The present invention generally relates to a digital image processing apparatus, and in particular, to a digital image processing suitable for halftone image representation based on the tone production method in which tone or gradation is represented by changing the ratio of recording dots to non-recording dots in unit area. The present invention is suitably applied to a digital color copier, a facsimile machine and the like.
A digital image processing apparatus is designed to process image data in digital form. For example, it is known to represent digital image data by the dot matrix method. In the dot matrix method, each dot is represented with only a few tone (gradation) levels. However, in order to record high-quality image relating to photographs or pictures on a recording medium such as paper and a print plate, a dot should be represented with 64 tone levels for each of the basic colors such as yellow (Y), Magenta (M), cyanogen (C), and black (Bk).
In order to make possible the above-mentioned multilevel image representation, a proposal is known where a dot area which has a fixed area and is composed of a plurality of dots, is defined as a unit area for the tone production process, and the tone representation is made by changing the number of recording dots and the number of non-recording dots contained in the unit area. Those examples of this type are a tone production method by density pattern, and a dither method.
However, the conventional multilevel image representation has a disadvantage in that as the number of tone levels increases, the resolution is deteriorated An increase of the number of tone levels is suitable for images of photographs, and on the other hand, is unsuitable for character images and line images, which are desired to reproduce images with high resolution. Therefore, it is preferable to represent character and line images by using a bi-level signal.
In the conventional dither method, it is known to use different dither matrix patterns based on which one of the tone representation and resolution should be mainly considered. When great importance is attached to the number of tone levels (tone representation), a dot concentration pattern (spiral type) is used. On the other hand, when great importance is attached to the resolution, a dot scattering pattern (Bayer type) is employed. The dot concentration pattern is of a relatively large matrix size, and the dot scattering pattern is of a relatively small matrix size.
An improved image processing has been proposed based on the above-mentioned dither method. In the proposed image processing, two halftone image processing modes are used. The optimum processing mode is selected from the two image processing modes, depending on the type of a document to be processed. For example, when a multicolored document such as photographs is processed, a dot concentration pattern is selected to effectively reproduce each color. When a document having dot images is processed, a smoothing process is carried out prior to the dither process in order to prevent the occurrence of moire. When a document having character images or line images is handled, an edge emphasizing process is performed, which is followed by the dither process where the dot scattering pattern is selected.
In order to apply the above-mentioned improved image processing to a mixed document having photographs, characters, dot images and line images, for example, it is necessary to extract features of an area of concern on the document and select the image processing most suitable for the extracted features of the area. Some proposals for identifying the type of document are known For example, it is known to automatically extract edges of characters or lines and select one of two halftone image processing modes based on the results of the edge detection (see Japanese Laid-Open Patent Application Nos. 62-186663, 62-186664, and 62-186665). In the proposal, when an edge is detected, an image processing mode is selected which provides high resolution. On the other hand, when no edge is detected, an image processing mode is selected which provides high gradation.
However, it is very difficult to reproduce a high-quality image by simply detecting the presence of edges of characters or lines. In other words, it is very difficult to represent various features of areas on an document to be scanned by simply detecting the presence of edges of characters or lines.